Kaposi's sarcoma (KS)-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV-8), was originally isolated from a KS lesion. KS is a vascular tumor of the skin that is the leading neoplasm of AIDS patients. It is now clear that KSHV is consistently found in all forms of KS (classical, endemic, post transplant, and AIDS-related), resulting in the widely held view that KSHV is the etiologic agent of KS. Infected cells in KS lesions include endothelial cells, spindle cells and immune infiltrating cells. KSHV infection of human endothelial cells causes long-term proliferation and survival of these cells. Inflammatory cytokines and innate immune responses are critical components of KS lesions facilitating paracrine-mediated promotion of angiogenesis and lytic versus latency states in infected cells. The mechanism by which KSHV activates inflammatory and innate immune responses is unknown. Recent discoveries from our laboratory have revealed that innate responses are triggered by envelope glycoproteins during, or in concert with, virus entry. The goal of this project is to define the initial interactions between KSHV envelope proteins and endothelial cell molecules and to characterize the immediate and long-term consequences of these interactions. The central hypothesis of this project is that interactions between KSHV envelope proteins with the microvascular endothelium trigger signal transduction pathways that lead to activation of innate and inflammatory immune responses characterized by substantial reprogramming of cellular transcription. Preliminary data show that purified KSHV K8. 1 glycoprotein activates a key mediator of interferon induction and induces an antiviral state in cells. This finding lends fundamental support to the overall hypothesis. To characterize alterations in cellular gene expression as a consequence of KSHV:cell interactions, transcriptional profiling and direct assessment of induced key innate immune activators will be measured. Other experiments are designed to elucidate the cellular machinery involved in transmission of extracellular-derived signals by KSHV envelope proteins. The final objective of this project is to identify the initiating mechanism of the iinnate activation responses. The relative contribution of known KSHV cell binding partners, heparan sulfate proteoglycans and alpha3beta1 integrins will be evaluated. In addition, we will directly test the hypothesis that KSHV is subject to innate sensing by Toll-like receptors, ancient defense molecules that transmit activation responses. Successful completion of these research goals will yield new insights into KSHV pathogenesis/tumorigenesis as well as have implications for future vaccine design.